Characterization of crystal structure. The crystal structure was determined based on the X-ray diffraction spectrometer (Panalytical Emperean) by using Cu K\(_{\alpha }\)  radiation with wavelength of 1.5406 Å. The polyimide film was used to cover the sample to avoid the oxidation of K\(_{3}\) C\(_{60}\) . The background signals of the film were carefully subtracted in the subsequent analysis. The space group and lattice parameters were determined by using \(\emph {Jana 2020}\)  program [1]} based on the Le Bail method [2]} to fit the diffraction patterns.
